Field of the Invention
The present invention relates to workholding, or clamping devices for simultaneously holding several workpieces on a machine tool, and more particularly to such a device which is able to hold one or several workpieces in place whilst it still has a pair of open jaws, so that the operator may insert one or several further workpieces between these jaws and simultaneously tighten the clamping device until it securely grips all workpieces.
Clamping devices can be mounted on the table of a machine tool and comprise vise-like jaws for fixedly maintaining one or several workpieces to be machined by the machine tool These jaws--hereafter called endjaws--are joined by a threaded spindle which can be tightened by rotating it in order to maintain at least one workpiece in a determined position relative to the machine tool. In order to speed-up the machining process, particularly on semi-automatic or automatic machine tools, it is known in the art to mount between the two end jaws of the clamping device a third jaw which is fixed relative to the main body of the device. One can then simultaneously clamp at least two workpieces in the device, by inserting one workpiece between each of the end jaws and the middle jaw. If one utilizes jaw plates, or shoes, which exactly match the shape of the workpieces, one can even maintain several workpieces between each of the end jaws and the middle jaw. Because this latter possibility remains unchanged for the invention, it will not be mentioned explicitly in the sequel; for simplicity only the case where merely two workpieces an clamped in the device, one on each side of the middle jaw will be described. An extension to the case of more workpieces is straightforward.
Now a problem occurs because when one progressively tightens the spindle, both jaws grip their respective workpiece practically simultaneously. If these pieces do not remain the desired positions, either through gravity or because of their shape, they must be manually maintained in the proper position until they are securely clamped. But one must also tighten the spindle, which practically requires three hands: two for maintaining the workpieces in place and another one for rotating the spindle. This difficulty occurs mainly when the clamping device is used in the upright position, i.e. when its spindle is oriented vertically and its jaws are positioned one above the other.
It is known in the art to provide clamping devices comprising three jaws, or on which a third jaw can be added between the end jaws, with a so-called "jaw brake", i.e. with a mechanism that allows to maintain with a sufficient force a workpiece between one endjaw and the middle jaw even when there is no workpiece between the latter and the other end jaw. The operator than has a hand free for inserting the second workpiece and maintaining it in place until he has tightened the spindle so that both workpieces are safely clamped. Conversely, this mechanism allows the operator to unload the clamping device by loosening the spindle with one hand and taking out the second workpiece whilst the first workpiece is still held by the pressure of the jaws associated with it. The ability to load and unload a clamping device in this manner is particularly useful when it must be used in a vertical position and workpieces not positively maintained by the corresponding jaws tend to drop out of the clamping device.
A known jaw brake consists of a mechanism affixed at the far end of the spindle, i.e. at its end not acted upon by the operator, on the rear side of the device.
This mechanism can be displaced axially with respect to the main body of the clamping device and has a liner slipping clutch which acts as a brake against such an axial displacement. It further comprises a compression spring one end of which remains immobile with respect to the main body of the clamping device as long as the clutch does not slip. The other end of the spring is fixed relatively to the rear end jaw. If now the clutch is in an appropriate axial position, a workpiece placed between the middle jaw and the front end jaw, and then the spindle progressively tightened, the spring will stop the rear end jaw from moving towards the middle jaw until the workpiece is clamped. Once it has been clamped, a further tightening of the spindle generates an increasing pressure on it and maintains it in place whilst the second workpiece is being inserted. After this the spindle can be fully tightened. If the second workpiece is so small that the stroke of the spring is exceeded before said workpiece is clamped, the clutch will slip and thus protect the device from damage.
Although such brakes are fully functional, they have a number of drawbacks. Adjusting their axial position and securing it by activating the linear clutch is troublesome and must be done at the far end of the device, furthest away from the operator. It is moreover necessary to remove the rear end jaw in order to activate the brake, and a similar difficulty arises when one wishes to regulate the force of the clutch which must usually be done by adjusting a screw by an unknown amount. Further, the overall dimensions of the brake are necessarily small because a part of it must be located in the recess intended for the spindle and may therefore not exceed the diameter of the letter. This enforced small size limits the dimension of the compression spring and hence its gripping power, but also the robustness of the entire brake.